In some communication systems, particularly systems in which communication terminals can be mobile, communication protocols are used in which ranging information relating to the range or distance between a first communication terminal and a second communication terminal is provided in communications between the terminals. Such information is considered to be important since it can be used to synchronize communications in the system as described later.
Examples of systems which employ a protocol in which ranging information is communicated in this way include systems which are proposed to operate in accordance with the 802.16e standard of the IEEE (Institute of Electrical and Electronic Engineers). The 802.16e standard of the IEEE, herein referred to as the ‘802.16e standard’ is an amendment to the 802.16 standard of the IEEE, herein referred to as the ‘802.16 standard’ to extend its applicability. The 802.16 standard entitled ‘Air Interface for Fixed Broadband Wireless Access Systems’ is the standard which was published by the IEEE on Apr. 8, 2002. It was developed by the 802.16 Working Group of the IEEE working on fixed broadband wireless access in Wireless Metropolitan Area Networks (WMAN). The 802.16 standard defines fixed terminal, point-to-multipoint, communications by BWA (Broadband Wireless Access). The 802.16e standard is the standard which was published by the IEEE on Feb. 28, 2006 entitled ‘Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands’. It extends operation of the 802.16 standard to wireless broadband connectivity by mobile terminals. The expression ‘802.16e standard’ as used herein includes this published standard and any future amendments or successions to the 802.16e standard published by the IEEE (or any successor standards authority).
Operation according to the 802.16e standard involves use of a form of OFDM modulation to communicate information. OFDM (Orthogonal Frequency Division Multiplexing) is a spread spectrum technology which allows high speed transmission of data via multiple lower speed sub-channels provided by division of the allocated frequency spectrum into sets of modulated sub-carriers.
The form of OFDM used in the protocol defined in the 802.16e standard is OFDMA (Orthogonal Frequency Division Multiple Access'). An OFDMA system is one in which different user terminals operate in the same frequency spectrum and each of these terminals occupies a separate channel.
In OFDMA communications, the available communication resource can be considered as a two dimensional entity and can be represented graphically by a two dimensional map. One dimension represents time and the other dimension represents frequency. Referring to the frequency dimension, the OFDMA sub-carriers are pseudo randomly spread on the entire available frequency spectrum for achieving frequency diversity. A designated group of spread sub-carriers is known as a frequency sub-channel. The time dimension is numbered (counted) in units of symbols, known also as OFDMA symbols. A given number of symbols in the time dimension makes up a frame.
In an OFDMA system the communication resource available is divided between user terminals by assigning a specified set of multiple sub-channels and multiple symbols per user terminal. Thus, the channel occupied by each user terminal is defined in terms of a specified time in which the user terminal occupies a specified sub-set of the sub-carriers defining a specified sub-channel for the specified time.
OFDMA systems such as those operating in accordance with the 802.16e standard require accurate synchronization between communicating terminals. According to known proposals, such synchronization may be achieved as follows. A first terminal, for example one of a plurality of user terminals, sends to a second terminal, for example a base station serving the user terminals, ranging information allowing the second terminal to detect a range or distance between the first and second terminals. The ranging information is received by the second terminal and is used by the second terminal to estimate a timing adjustment needed to be applied by the first terminal. The second terminal sends information relating to the estimated timing adjustment to the first terminal. The timing adjustment is applied by the first terminal so that a signal from the first terminal can arrive at the second terminal at the same time as signals sent from other terminals, e.g. user terminals. Thus, where the second terminal is a base station, it may estimate in this way the range of each user terminal it serves and provide a timing adjustment to each served user station so that incoming signals, known in the art as ‘uplink’ signals, are received from all served terminals arrive together in synchronization.